FIELD OF THE INVENTION
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The present invention relates to transmission methods, apparatuses, and
systems for communication systems and, more particularly, to polarization alternating optical
transmission methods, apparatuses, and systems.
BACKGROUND OF THE INVENTION
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Polarization alternating of adjacent signal channels in wavelength division
multiplexed systems is a known technique to improve transmission performance. However,
polarization alternating has several drawbacks. For example, prior art polarization alternating
systems require numerous polarization maintaining components, connections, and splices.
Unfortunately, performance rapidly degrades as the number of successive polarization
maintaining components, connections, and splices increases. Furthermore, it is expensive and
difficult to manufacture systems containing a large number of polarization maintaining
components, connections, and splices associated with the prior art.
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Another problem with prior art polarization alternating is that it is difficult and
expensive to monitor and control the polarization of signals in the transmission lines. As a
result, after signals are transmitted, it is not practical to determine or control their
polarization within the transmission lines. Because the polarization cannot be determined or
controlled in a practical manner, there is no control over the polarization of signals added in
one part of the system relative to signals which are added in other parts of the system.
Because commercial transmission systems have many points at which signals are added and
dropped, prior art polarization alternating systems do not provide a practical solution to
provide the signals in the desired relative polarization, because they teach controlling relative
polarizations between all channels, homogeneously over the entire signal wavelength
division multiplexed optical band.
BRIEF DESCRIPTION OF THE INVENTION
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The present invention relates to transmission methods, apparatuses, and
systems for communication systems. One embodiment of the present invention is an optical
communications system including a first group of transmitters producing a first spectral
group containing at least two optical signals, wherein each of the signals in the first spectral
group has a polarization orientation which is non-parallel to adjacent signals within the first
spectral group. the system also includes a second group of transmitters producing a second
spectral group containing at least two optical signals, wherein each of the signals in the
second spectral group has a polarization orientation which is non-parallel to adjacent signals
within the second spectral group, and wherein the polarization orientation of the first spectral
group is independent of the polarization orientation of the second spectral group. The
polarization orientation of signals within each spectral group may be, for example,
orthogonal or other non-parallel orientations.
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The system according to the present invention may include two or more
transmitters producing polarized optical signals, a polarization maintaining coupler, and
polarization maintaining fibers connecting the transmitters to the coupler, wherein the
polarization maintaining fiber is connected to the transmitters and the coupler such that
optical signals at adjacent optical wavelengths have non-parallel polarization orientations.
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The present invention also includes methods of transmitting optical signals,
such as transmitting a first spectral group including at least two signals and wherein the
signals in the first spectral group have polarization orientations which are non-parallel to
adjacent signals in the first spectral group, and transmitting a second spectral group including
at least two signals wherein the signals in the second spectral group have polarization
orientations which are non-parallel to adjacent signals in the second spectral group, wherein
transmitting the second spectral group is independent of the polarization orientation of the
first spectral group.
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The present invention offers a cost effective way to improve performance in
optical communications systems without the disadvantages of the prior art. Those and other
advantages of the present invention will be described hereinbelow.
BRIEF DESCRIPTION OF THE DRAWINGS
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Embodiments of the present invention will now be described, by way of
example only, with reference to the accompanying drawings, wherein:
- Figs. 1 and 2 show examples optical communications systems;
- Fig. 3 shows a signal profile of several signal channels which form a spectral
group;
- Fig. 4 shows an embodiment of a sub-rack according to the present invention;
- Figs. 5-7 show a signal profile for two adjacent spectral groups; and
- Fig. 8 shows another embodiment of the present invention with several groups
of transmitters are utilized at the same location to transmit several spectral groups.
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DETAILED DESCRIPTION OF THE INVENTION
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Fig. 1 illustrates an optical communications system 10 which includes optical
paths 12 connecting nodes and network elements 14. Advantages of the present invention can
be realized with many system 10 configurations and architectures, such as an all optical
network, one or more point to point links, one or more rings, a mesh, other architectures, or
combinations of architectures. The system 10 illustrated in Fig. 1 is a multi-dimensional
network, which can be implemented, for example, as an all optical mesh network, as a
collection of point to point links, or as a combination of architectures. The system 10 can
employ various signal formats, and can also convert between formats. The system 10 can
also include more or less features than those illustrated herein, such as by including or
deleting a network management system ("NMS") 16 and changing the number, location,
content, configuration, and connection of nodes 14.
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The optical paths 12 can include guided and unguided transmission media,
such as one or more optical fibers, ribbon fibers, planar devices, and free space devices, and
can interconnect the nodes 14 providing optical communication paths through the system 10.
Various types of transmission media can be used, such as dispersion shifted ("DSF"), non-dispersion
shifted ("NDSF"), non-zero dispersion shifted ("NZDSF"), dispersion
compensating ("DCF"), polarization maintaining ("PMF"), single mode ("SMF"), multimode
("MMF"), other types of transmission media, and combinations of transmission media.
Furthermore, the transmission media can be doped, such as with erbium, germanium,
neodymium, praseodymium, ytterbium, other rare earth elements, other dopants, and
mixtures thereof. The paths 12 can carry one or more uni- or bi-directionally propagating
optical signal channels or wavelengths. The optical signal channels can be treated
individually or as a single group, or they can be organized into two or more wavebands or
spectral groups, each containing one or more optical signal channel. The optical signal
channels within a spectral group are all treated the same. For example, all optical signal
channels in a spectral group are switched in the same manner, and all are dropped at the same
locations, even if every optical signal channel in the spectral group is not utilized at every
location at which it is dropped. The use of spectral groups to treat groups of channels in the
same manner is one way to efficiently manage large numbers of optical signal channels. One
or more paths 12 can be provided between nodes 14 and can be connected to protection
switching devices and/or other redundancy systems. The optical path 12 between adjacent
nodes 14 is typically referred to as a link 18, and the optical path 12 between adjacent
components along a link 18 is typically referred to as a span.
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The nodes and network elements 14 can include one or more signal processing
devices including one or more of various optical and/or electrical components. The nodes 14
can perform network functions or processes, such as switching, routing, amplifying,
multiplexing, combining, demultiplexing, distributing, or otherwise processing optical
signals. For example, nodes 14 can include one or more transmitters 20, receivers 22,
switches 24, add/drop multiplexers 26, amplifiers 30, interfacial devices 28,
multiplexers/combiners 34, and demultiplexers/distributors 36, as well as filters, dispersion
compensating and shifting devices, monitors, couplers, splitters, and other devices. One
embodiment of one node 14 is illustrated in Fig. 1, although the nodes 14 can have many
other variations and embodiments.
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The NMS 16 can manage, configure, and control nodes 14 and can include
multiple management layers that can be directly and indirectly connected to the nodes 14.
The NMS 16 can be directly connected to some nodes 14 via a data communication network
(shown in broken lines) and indirectly connected to other nodes 14 via a combination of a
directly connected node and communications paths in the optical system 10. The data
communication network can, for example, be a dedicated network, a shared network, or a
combination thereof. A data communications network utilizing a shared network can include,
for example, dial-up connections to the nodes 14 through a public telephone system. The
NMS 16 can reside at one or more centralized locations and/or can be distributed among
components in the system 10. Mixed data or supervisory channels can be used to provide
connections between the network elements of the NMS 16, which can be located in nodes 14
or remote from nodes 14. The supervisory channels can be transmitted within and/or outside
the signal wavelength band and on the same medium or a different medium than the
wavelength band.
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Fig. 2 illustrates another embodiment of the system 10 including a link 18 of
four nodes and network elements 14. That system 10 can, for example, be all or part of a
point to point system 10, or it may be part of a multi-dimensional, mesh, or other system 10.
One or more of the nodes 14 can be connected directly to the network management system
16 (not shown). If the system 10 is part of a larger system, then as few as none of the nodes
14 can be connected to the network management system 16 and all of the nodes 14 can still
be indirectly connected to the NMS 16 via another node in the larger system 10.
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Fig. 3 illustrates a signal profile of several signal channels which form a
spectral group and which are polarization alternated. The signal channels within the spectral
group are polarization alternated such that each signal channel within the spectral group has a
polarization which is orthogonal to that of the adjacent channels in the spectral group.
However, unlike prior art systems, the polarization orientation of signal channels in one
spectral group are not controlled relative to the polarization orientation of signal channels in
all other spectral groups. Nonetheless, polarization alternating according to the present
invention improves transmission performance and capacity, such as by reducing non-linear
effects occurring between channels, thereby allowing channels to be closer together in the
frequency domain. Although the present invention will be described in terms of polarization
alternating in which the signals are orthogonal, advantages of the present invention can be
realized by polarization alternating signals at orientations other than orthogonal. For
example, each successive channel may have a polarization orientation offset by 120 degrees
from adjacent channels, so that the same polarization orientation is used every three
channels. Other variations are also possible depending on the particular application, such as
using different polarization orientations and mixing polarization orientations. Furthermore,
although the present invention is described in terms of repeating, evenly-spaced polarization
alternating patterns, it is possible to realize benefits of the present invention with irregular
polarization alternating patterns.
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Fig. 4 illustrates one embodiment of a "sub-rack" 40 according to the present
invention which can be used to produce polarization alternated signals. In that embodiment,
several transmitters 20 are connected to a polarization maintaining coupler 34 via
polarization maintaining fiber 12. The invention may also utilize polarization maintaining
wavelength division multiplexers or similar devices in place of the couplers. The transmitters
20 produce polarized optical signals, and the polarization of the optical signals is maintained
in a known orientation in the polarization maintaining fiber 12 and the polarization
maintaining coupler 34. Polarization alternating can be affected by rotating some of the
polarization maintaining fiber 12. For example, the polarization maintaining fiber 12
corresponding to the odd numbered transmitters 20 can be rotated ninety degrees relative to
the fibers 12 corresponding to the adjacent, even numbered transmitters 20. The rotation can
be done at the transmitters 20 or at the inputs to the coupler 34. As a result, the polarization
of the signal channels will possess the desired polarization when combined in the
polarization maintaining coupler 34. After the signals are coupled, they will retain their
relative polarization, even if they are traveling in non-polarization maintaining fiber 12 or
devices.
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Signals channels can be added and/or dropped at various locations within the
system 10. Typically, most of the fiber 12 in a system 10 is not polarization maintaining
fiber. As a result, optical signals will rotate within the fiber 12, although signals which are
transmitted together will retain their polarization orientation relative to each other. However,
it is difficult to predict the orientation of those signal channels relative to the fiber 12 or
relative to other signal channels which are added at another point in the system 10. In the
present invention, signal channels are added and dropped in spectral groups, and polarization
alternating is performed on channels within each spectral group. However, the relative
polarization of adjacent spectral groups is not controlled. As a result, channels at the edge of
adjacent spectral groups can have a relative polarization orientation which varies from
parallel to orthogonal. If the channels at the edge of adjacent spectral groups have a parallel
polarization orientation, the transmission performance for those channels will not be as good
as when the polarization orientation is orthogonal. Such performance degradation can be
mitigated by providing additional spacing between spectral groups, sometimes called a
"guard band". Even without additional spacing between spectral groups, however, only the
adjacent channels at the edges of the spectral groups are at risk of being parallel with an
adjacent channel, which still provides for superior overall transmission performance than is
the case when polarization alternating is not employed. In most cases, however, there will be
at least some non-parallel polarization orientation with all adjacent channels.
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A further advantage of the present invention is that the number of polarization
maintaining connections and splices is significantly reduced. Typically, only two or three
polarization maintaining connections are required in each signal path. For example, two
polarization maintaining connections are used in the embodiment illustrated in Fig. 4, one
polarization maintaining connection between the fiber 12 and the transmitter 20, and one
polarization maintaining connection between the fiber 12 and the coupler 34. Additional
polarization maintaining connections may be used, for example, if more than one polarization
maintaining coupler is used to couple the signals forming a single spectral group, or if the
relative polarization of more than one spectral group is to be controlled. The later example
may be advantageous if, for example, more than one spectral group is being added at the
same location, such that alternating polarization can be controlled over more than one
spectral group without the need to determine the relative polarization of signals in non-polarization
maintaining fiber 12.
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Fig. 5 illustrates a signal profile for two adjacent spectral groups which are
added to the system 10 independent of each other. In that example, the polarization
orientation of the adjacent signal channels at the edges of the spectral groups happens to be
parallel. Fig. 6 illustrates a signal profile for two adjacent spectral groups which are added to
the system 10 independent of each other, and in which the polarization orientation of
adjacent signal channels at the edges of the spectral groups happens to be orthogonal. Fig. 7
illustrates a signal profile for two adjacent spectral groups which are added to the system
independent of each other, and in which additional spacing, a guard band, is provided
between the spectral groups. The guard band mitigates performance degradation in the event
channels at the edges of the spectral groups happen to be parallel. The guard band may be,
for example, the equivalent of one channel spacing. More or less spacing may also be
utilized, depending on the particular application and the performance required.
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Fig. 8 illustrates another embodiment of the present invention in which several
groups of transmitters 20, such as those illustrated in Fig. 4, are utilized at the same location
to transmit several spectral groups. For clarity, the illustrated embodiment shows transmitters
20 at only one location, although transmitters 20 are typically located at several locations in
the system 10. That embodiment illustrates a transmit portion of a system which is modular
at the spectral group level, according to the teachings of the present invention. For example,
each sub-rack 40 produces a spectral group, and optical signals within each spectral group
are polarization alternated on a spectral group level in the sub-racks 40, as described above.
Other processing can also be performed on the spectral group level, such as filtering,
attenuation, amplification, etc. Furthermore, signal processing may also be performed in a
modular manner in groups of two or more spectral groups having similar characteristics.
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For example, sub-racks 1-4 produce spectral groups 1-4. Optical signals from
spectral groups 1-4 are filtered and attenuated at the spectral group level before being
coupled together. After being coupled together, spectral groups 1-4 are processed together,
for amplification, filtering, attenuation, and dispersion compensation. Similar modularity is
applied to other spectral groups in Fig. 8, and lower level modular sections are combined to
form higher level modular sections until all of the spectral groups are coupled together.
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This modularity simplifies signal processing and allows for more precise
treatment of optical signals. For example, each dispersion compensation stage 42,
individually labeled DCF1-DCF8, in Fig. 8 can be tailored to the spectral groups passing
through that stage, without regard to the effect that the dispersion compensating stage might
have had on other spectral groups, thereby providing a differentiated dispersion
compensation approach on a spectral group basis, or on a basis of several combined spectral
groups. Other differentiated processing may also be performed, such as amplification,
filtering, and attenuation.
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For example, the design of the transmission site in Fig. 8 is modular at the
spectral group level between the sub-racks 40 and the 4:1 couplers 34, and is modular in
groups of four spectral groups between the 4:1 couplers 34 and the 2:1 couplers 34. This
modularity simplifies the design and makes it easier to change a design to add and remove
spectral groups. Differences exist in the modular sections to accommodate specific
characteristics of the spectral groups. For example, different spectral groups may pass
through different numbers of couplers 34 and, thereby, experience different amounts of
attenuation. Those and other variations can be addressed by modifying characteristics of the
different modular sections. For example, proper selection of dispersion compensating fiber
42 can offset attenuation differences in the signal paths.
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Spectral group modularity is also applicable to other parts of the system, such
as receivers and add/drop multiplexers. For example, by dropping signals in spectral groups,
the drop multiplexers and receivers can utilize modular designs analogous to that of the
transmitter 20 in Fig. 8.
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In the illustrated embodiment filters and variable attenuators may be used to
groom the signals. Dispersion compensating fiber may also be provided to compensate for
chromatic dispersion introduced by the system. The signals may be further combined in
several steps until all of the signals are on a single fiber. Control lasers 44, amplifiers 30, and
"pre-chirp" devices may also be used to prepare the signals for transport in the system 10.
